The essence of irony is opposition, or the presence of opposites. In Shakespeare's Macbeth, any contrast between appearance and reality is ironic. Needless to say, irony is prevalent in the play.
This theme is first mentioned in Act 1.1 when the witches introduce the fair is foul and foul is fair thought. Macbeth echoes them later, of course, and the discrepancy between appearance and reality reoccurs numerous times during the play.
- The Thane of Cawdor, says King Duncan, was a man he put complete trust in, but he ended up being a traitor.
- Macbeth also receives the complete trust of the king, but then assassinates him.
- Lady Macbeth plays the perfect hostess, while simultaneously planning Duncan's death.
- While on stage with her husband, Lady Macbeth appears hardened and aggressive, yet in private feels the need to plead with her spirits to be made more like a ruthless warrior, and later suffers a breakdown stemming from her guilt.
- Malcolm doesn't know whether or not he can trust Macduff--he thinks Macduff might be Macbeth's agent. Even the honest are suspected of being teacherous.
And so forth. As catalyst for the entire plot, the witches manipulate Macbeth, equivocating by telling him half-truths and suggestive predictions. Macbeth assumes the witches are being straight with him, but of course they are not.